In coming years, the proportion of motor vehicles, which are operated with alcohol, will go up increasingly. In this way, alcohol, e.g. ethanol may be present in fuel in different concentrations of between 0 and 100%. Such fuels are also referred to as “FlexFuel” or “Flexible Fuel” (FF).
The basic operation of motor vehicles with alcohol was already proven a long time ago and many vehicles are provided with alcohol for operation particularly in South America and North America. The concentration of alcohol in the tank fuel may vary from one tank filling to another tank filling depending on the filled fuel. Any concentrations of alcohol in the fuel may result in this way. For a faultless operation of the combustion engine, it is necessary for the operating control device of the combustion engine to identify the new fuel composition as soon as possible and to take this into account in its control strategies.
In the document U.S. Pat. No. 6,257,174 B1, a method is described in which the alcohol concentration of the new fuel mixture is estimated by way of the ratios of the volumes of the fill level quantities before and after filling as well as the fact that only pure gasoline or flex-fuel of an alcohol concentration of 85% could have been filled. This means that when pure gasoline is filled, the alcohol concentration will drop and when E85 fuel is filled, the alcohol concentration will remain the same or increase. The previously known method takes advantage of the fact that the lambda value (air/fuel ratio) measured by the lambda sensor changes if the alcohol concentration of the fuel fed to the combustion changes. With the previously known method, based on the fuel located in the tank, an upper and a lower threshold are thus predetermined for the lambda value, so that if the respective threshold is exceeded or not reached, the filled fuel type can be indicated. The engine timing is then operated taking the new alcohol concentration of the fuel into account.
With this method, there is however the problem that the new fuel mixture is not instantly available at the injection point immediately after filling, because “old” fuel with the previous alcohol concentration is generally located in the fuel supply line and in the fuel filter. This also results in the “new” and “old” fuel mixing, with it being possible for this mixture initially to be different to the mixture of the fuel in the tank as a result of different volume ratios. For this reason, “change-over mixture ratios” exist as long as the new fuel has displaced the old fuel completely.
As already mentioned, it is extremely important to determine the fuel mixture as soon as possible after filling because certain system diagnoses like for instance the diagnosis of the fuel system prescribed by the lawmaker are dependent on admissible lambda values.
When determining the alcohol concentration of the tank fuel after filling, account must also be taken of the measured lambda values not only being changeable as a result of the alcohol proportion but instead also as a result of components in the fuel system ageing. For instance, the flow rates of the injection valves may change throughout the service life. It is already known in the prior art that the operating control device of the combustion engine takes corresponding ageing effects into account by means of adaptation methods, with deviations in the lambda values of for instance ±25% from the standard values being allowed.
During operation of the combustion engine with fuel of different alcohol concentrations, there is however the problem that the operating control device can no longer determine whether changes in the lambda value were caused by components aging or by changes in the alcohol concentration.